A display device which is worn on a head for viewing images, in other words, a head-mount display (HMD), has been widely known. Such a head-mount display is configured to have optical units each for the left and right eyes, and to be able to control the senses of viewing and hearing by also using a headphone. If such a device is configured to completely block the outside world when mounted on a head, a feeling of virtual reality during viewing increases. In addition, as a head-mount display can also project different images to the right and left eyes, 3D images can be presented when images having parallax are displayed for the right and left eyes.
For a display unit of a head-mount display for the right and left eyes, high-definition image display elements which include, for example, liquid crystal or organic EL (Electro-Luminescence) elements, or the like can be used. In addition, if a wide angle of view is set by enlarging and projecting image display elements with an eyepiece optical system, and multi-channels are realized using a headphone, a rich feeling of presence in a movie theater for viewing and listening can be reproduced.
An optical lens has been known to have distortion. For example, if the angle of view is taken in the head-mount display, complex distortion and color deviation are generated in an observed image caused by distortion of the lens used in the eyepiece optical system, and thus, there is concerned that quality is deteriorated.
If the number of lenses constituting the eyepiece optical system is increased in order to ensure the wide angle of view, the weight of the head-mount display is increases so that more burdens are imposed on the user wearing the head-mount display. Here, if the number of lenses is decreased for weight saving, the distortion generated in each lens is enlarged and a system for correcting the distortion is lost. As a result, the angle of view is difficult to be widely taken.
There has been known a method for correcting the distortion generated in the eyepiece optical system by a signal process. For example, there has been proposed a display device in which a size of the image to be displayed on the image display elements is varied for each wavelength such that chromatic aberration of magnification generated in an optical system is cancelled in advance, and therefore, the observed image can be observed as if with no chromatic aberration of magnification (e.g., see Patent Literature 1).
This display device compensates a phenomenon in a positive lens system that since the longer wavelength a light ray has, the weaker a refractive index, and the shorter wavelength the light ray has, the stronger the refractive power, an image of a red color having the long wavelength is observed small and an image of a blue color having the short wavelength is observed widely. In short, in this display device, an image having sizes different for each color is passed through an optical system having the chromatic aberration of magnification such that light fluxes of respective colors are made to approximately coincide one another at an incident position on an eyeball in the case of the eyepiece optical system and at an incident portion on a screen or the like in the case of a projection optical system, and therefore, the observed image can be observed as if with no chromatic aberration of magnification.
However, the distortion generated in the image upon passing through the eyepiece optical system is not limited to simple one due to the differences in the refractive powers depending on the wavelength. Simply varying the size of the image (display ratio) for each wavelength (a color of long wavelength is displayed smaller than a color of short wavelength) cannot eliminate an image distortion due to a complex phenomenon such as a lens distortion.
Examples of another signal processing method for correcting the distortion generated in the eyepiece optical system include address translation. In other words, a distorted image is moved to an ideal image position on the basis of a correspondence relationship between an image location obtained by an ideal optical system and an image location affected by an actual aberration. Here, in a case where an image size is large or high translation accuracy is required, disadvantageously a size of a correction table storing a correspondence relationship with coordinates after translation is bloated. For example, there has been proposed a method in which, by use of a feature that the distortion in the optical system is point-symmetric with respect to an image (rotationally symmetric with respect to an optical axis), not a correction data for an entire image but a correction data for an area of half or quarter of the image is used also for other symmetric area (e.g., see Patent Literatures 2 and 3). However, the size of the correction table may be reduced with assumption that the display image distortion is rotationally symmetric with respect to the optical axis. In other words, this cannot be applied to the distortion which is not rotationally symmetric with respect to the optical axis.
Further, there has been proposed an image display system which includes an image display device having the eyepiece optical system and a control device outputting to an image display device an image signal containing distortion inverse to optical distortion which is generated in the image due to the eyepiece optical system, and in which the control device side stores reference data as representative correction data calculated from a design value or the like, the image display device side stores difference data as the correction data corresponding to an individual difference of the optical system, and the control device uses the reference data and the difference data to generate the image signal containing the inverse distortion (e.g., see Patent Literature 4). According to this image display system, amounts of the correction data stored respectively in the control device and the image display device can be reduced and a time for the control device to read the correction data from image display device can be reduced. However, it is remained that the correction data for the entire image is held.